Power over Ethernet (PoE) technology generally allows existing cables to transmit data as well as electric power without changing the existing Ethernet architecture. At present, the PoE technology has been approved by the Institute of Electrical and Electronics Engineers (IEEE), and the IEEE 802.3af standard is established for specifying the items related to power detection and control of a network power supply system.
In the IEEE 802.3af standard, a power source device of a network power supply system must detect whether or not the equivalent resistance and the equivalent capacitance of a powered device (which is referred to as a capacitive load circuit) meet the specifications of the IEEE 802.3af standard and output the power to the powered device when the powered device is in compliance with the PoE standard. However, it is difficult to accurately measure the value of the equivalent capacitance of the powered device, so the conventional method only measures the equivalent resistance of the powered device. Therefore, the performance of the conventional method is not perfect.